  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' 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.
