The Answer: There are many variables, but for sure below 1mOhm….
The magnitude depends on the VNA used, the cables, the ground isolator and the calibration as well as the probe attenuation. Using PDN Cables, the Bode 100, J2113A ground isolator and a 1X P2102A probe, the impedance floor below is about 30uOhms and 60pH. This is using 2-port SOL calibration, 13dBm source level and 30Hz receiver bandwidth. Theoretically a 100uOhm impedance and 180pH could be measured with reasonable accuracy. I wouldn’t specify the probe for 100uOhms, but in this example, I measured 250uOhms. The DUT is a 250uOhm Stackpole CSS2725 resistor.
First, it’s essential to know the correct answer. I used the 2-port probe to measure the resistor at DC. I did this by forcing 1 amp through one port and measuring the voltage, using an accurate DMM, at the other port. The correct resistance is 275uOhm. The Bode 100 measures 276uOhms and so the probe accurately measured the 275uOhm resistor. The inductance is a bit trickier, since we don’t actually know the inductance of the short. The incremental inductance for the resistor is likely the difference between the measurement and the short. To measure this inductance accurately requires using a calibrated calibrator, or alternatively a 3D simulation of the short with the contact pins matching the probe contact point during calibration.