Conductance noise on a square sample, calculation of the geometrical factor fQ

Conductance fluctuations are easily probed by passing a constant current through a pair of driver electrodes and measuring the voltage fluctuations across a pair of sensor electrodes. We calculate the geometrical factor for these fluctuations from the current distribution derived by conformal transformations. In four-electrode configurations on a flat square sample this factor depends only slightly on electrode size, whereas a strong dependence is found for two-electrode configurations. The calculations also give accurate values for the resistance, which allows a comparison with the thermal noise.