Re-examination of quantum Hall plateaus

Even though the unit of electrical resistance was based on the quantum Hall effect starting January 1, 1990, the understanding of the fundamental physics of current flow, contacting, and impurity effects in quantum Hall systems remains incomplete. Some effects which may affect quantum Hall resistance determinations are examined. Improvements to the NIST potentiometric measurement system are described, and new data comparing the i=4 plateaus of a Si-MOSFET and a GaAs heterostructure with a room temperature reference resistance are presented.<>