Dependence of contact resistance on current for ohmic contacts to quantized Hall resistors

The dependence of contact resistance on current has been measured for a large number of ohmic contacts to quantized Hall resistors under quantum Hall effect conditions. Five different functional forms of current dependence are observed at low currents. The trend from best to worst quality can be correlated with the density of defects in the contact, regardless of the physical cause of the defects. The consequences of different types of contact resistance current dependence on the metrological use of samples are discussed