This is another log I moved out of the quick comments because it's starting to snowball.

For a point of reference, the resistivity of pure/bulk metals are approx:

• Copper: 2 μΩ cm
• Brass: 4 μΩ cm
• Iron: 10 μΩ cm
• Stainless: 90 μΩ cm

Finding "Copper Composites and Laser Sintering: Novel Hybridization Method for 3D Printed Electronics"

On April the 24th, I found this open-access paper that's 2 years old: 

• https://advanced.onlinelibrary.wiley.com/doi/full/10.1002/admt.202201900

Two notable quotes are below:

PLA-based composites with 90% and 80% (wt.%) copper loading [with] a 5.5-W 450-nm blue laser with various combinations of parameters to “sinter” the surface of the samples. [...] The copper particles became oxidized during the process, and none of the samples yielded a conductive result. 

Instead of partial melting, sintering (i.e., bridging adjacent particles) can be achieved by chemical reaction that causes deposition of a native copper layer on copper particles [...] using copper formate, which [...] spontaneously decomposes at approximately 200 °C to form native copper and volatile compounds. [...] However, copper formate, formic acid, and ethylene glycol are all incompatible with the temperatures associated with FDM printing (200–250 °C).

They reported a resistivity of 400 μΩ cm and included a video showing a 5.5W blue laser adding a new trace to a printed, active circuit:

Finding loads more papers

1. One-Step Fabrication of Copper Electrode by Laser-Induced Direct Local Reduction and Agglomeration of Copper Oxide Nanoparticle (2011)
   1. 31 μΩ cm
   2. Laser: 1070nm 0.4W 25μm at 30mm/s
   3. "There are two types of copper oxides: CuO and Cu2O. The band gap of CuO (∼1.2 eV) is lower than that of Cu2O (∼2.1 eV). Hence, Cu2O is reported to have a high transparency with a slightly yellowish color and usually absorbs wavelengths below 600 nm, while CuO strongly absorbs the whole visible spectrum range and is black in appearance."
      1. So it sounds like a 450nm laser should be fine. I expect all the researchers are using lasers that their uni had on hand, and high powered blue lasers are relatively new.
2. Photonic Sintering of Composite Pastes with Copper Oxide Powders Using Different Light Sources (2021)
   1. 8 μΩ cm on PET substrate
      1. It seems only a single layer is formed in these experiments, so the substrate its on affects the resistance.
   2. Laser: 638nm 1W
3. Direct Writing of Semiconducting and Conducting Microstructures by Using Selective Laser Sintering and Reduction of CuO Nanoparticles (2024)
   1. 88 μΩ cm
   2. Femtosecond Laser: 800nm 0.04W at 0.004mm/s
   3. "The cupric oxide (CuO) nanoparticles are air stable, cheap, and easily available materials; they are also good precursors for fabrication of Cu micro/nanostructures by using selective laser reduction."
      1. So I guess this is why they're common in academia.
4. Fabrication of Flexible Copper Microelectrodes Using Laser Direct Writing for Sensing Applications (2024)
   1. 62 μΩ cm
   2. Laser: 1064nm 0.955W* at 60mm/s
      1. *This laser was defocused because it's minimum power was 10W for a 42μm spot size was too much.
5. Direct laser writing of copper and copper oxide structures on plastic substrates for memristor devices (2023)
   1. They only mentioned ohm/square measurements
   2. Laser: 1064nm 4W 15mm/s
6. Additive Manufacturing of Electrically Conductive Multi-Layered Nanocopper in an Air Environment (2024)
   1. 28 μΩ cm
   2. Laser: 1064nm 2.5W 50μm at 150mm/s