How to measure voltage drop across a wire?
Normally I would run a constant current from an external power supply and measure the voltage drop on the wire. If my current is 1A, the voltage indicates the exact resistance of the wire, so 4.6mV, for example, would mean 4.6mΩ.
If you can't measure below 0.1mV (100μV), you can increase the current to 10A (if your supply is capable) and that way measure a 10 times smaller resistance.  In this case, 4.6mV would indicate a 0.46mΩ resistance.