Measuring four point conductivity on a very small bar sample has two main challenges:

First, to calculate the volume specific conductivity or resistivity of the sample, the distance between the voltage electrodes (and other dimensions too) needs to be known. On a small sample, where these electrodes have a distance of few mm, the error, the "measured distance to actual distance"-ratio gets more unfavorable than on a large sample. Having measurement error of 0.2 mm with total distance of 3 mm is much more significant than same 0.2 mm measurement error on a sample that has voltage electrode distance of, say, 30 mm. This also applies to the cross section.

Similarly, all other sources of error remain same regardless of sample size, while the measured property of the sample get smaller as the sample get smaller. A larger sample just has better "signal to noise" ratio than a small one, so to speak.

One could argue that this first point has nothing to do with the sample holder, and be mostly correct about that, but regardless, we feel it is our duty to point this out to customer wanting to measure small samples.

Secondly, mounting these electrodes, with only few mm gap, is harder than mounting them with 30 mm gap. The electrodes should have the same distance from each other around the whole sample. This is naturally easier on a sample with larger diameter.

(For those unfamiliar with this measurement method, the two separate voltage electrode contacts are typically 0.5 or 0.25 mm diameter platinum wire, wrapped around the sample, one loop. These wires then connects to the two larger electrode contact wires, bit further from the sample. The current electrodes are pressed at the top and bottom of the sample, held in suspension with a spring load system.)

It is relatively easy to mount and measure samples down to 5 mm length, and 4 mm^2 cross section, but samples smaller than that require special attention that is beyond the scope of our standard support; a skilled user can try, and succeed, but with no guarantees from our side.

To measure bar sample with two point contact, for resistance or voltage or impedance spectroscopy, the sample length has no such meaning, but the cross section still should be at least 1 mm^2.