Contact resistance is usually in the 1-4mOhm mark depending on materials used.
In that case, gold is usually 1mOhm due to its high conducivity and softness, nickel is usually 2MmOhm, and steel can be even worse at up to 3,3-3,9mOhm, or even higher in some cases.
Or even worse. A copper alloy spring which is oxidized can be worse than that with no coating
This is why I chose a nickel plating: most cost effective while still having low contact resistance.
In most cases, like the BeCu spring I made, I chose mechanical properties over pure conductivity. I could have went with BeCu C17500 instead of BeCu C17530.
I could have gotten an even lower voltage drop at 0.045V at 6A had I changed the material, but at that point, it would be way worse mechanically, and a 0,007V difference is quite small.
That’s the whole point of why I made this specific spring: to get a spring good enough for 95% of cases where you don’t need more than 13-14A of current, and easy to bypass copper alloy spring.
Using higher conductivity parts is also crucial in FETs, but also regulated linear drivers, but most importantly, boost drivers.
TLDR: A nice phosphor bronze spring, or even BeCu spring if you want even higher power, is usually great in most cases.
If you want uttemost performance, while sacrificing mechanical properties, go with a spring bypass!