4 wire resistive measurements to cancel out the resistivity of the probes is a standard practice[1].

[1] https://www.ni.com/docs/en-US/bundle/ni-daqmx/page/measfunds...

The problem isn't the resistivity of the probes, it's that the probes may not be connected to the same crystal.

I understand the 4 wire idea, but the two inner wires are still farther apart than an individual crystal might be. So a material composed of 100 um superconducting crystals in a resistive bulk medium would appear resistive to probes that are 1 mm apart.

Got it. Building a 4 wire 100um square probe would be doable certainly but not off the shelf.

Where have you seen them soldered? That would be very strange.

https://arxiv.org/pdf/2308.01192.pdf in figure 3.

I'm not sure how they're attached (it looks like gold wire bonding) but the distance is a few mm.

Right, thank you for digging that up. It looks like you are right that is soldered. Either that or wire bonded which is usually a weld, but welding would locally raise the temperature to the point that I wonder if that wouldn't potentially destroy the sample. Given the tiny current I wonder why they didn't use spring loaded contacts, but presumably they know what they are doing and this is just another 'why didn't they' style comment.

So many questions about all of this, I would love to interview all these people, but I totally get that they have much better things to do right now.