Abstract
Two distinct models account for the microwave residual surface resistance of superconducting cavities with equally good agreement with the measured temperature and frequency dependence. In presenting his phonon-generation model. Passow claimed that Rabinowitz's fluxoid power-loss model of residual resistance does not fit the experimental data, whereas his does. In fact, the two models have essentially the same temperature and frequency dependence. Furthermore, Passow's phonon-generation model cannot explain the observed sensitivity to details of sample preparation and history while the fluxoid model can.