Quantum Hall measurements from 4 K to 20 mK

We report on precision measurements of the quantum Hall resistance (QHR) in the temperature range of 20 mK to 4 K. These include intercomparisons of the ohm maintained at the National Research Council (OHM NRC) and the International System ohm (OHM SI) and the QHR. The effects of nonzero Rxx minima, sample inhomogeneity, nonohmic contacts, and dissipation are summarized.