  In the conventional BCS description of a superconductor the kinetic energy
increases in the superconducting state. We describe the observed decrease in
kinetic energy by adopting a simple model of electrons whose elastic scattering
rate undergoes a sharp decrease as the temperature is lowered below Tc. This
phenomenology has been suggested by other experiments, particularly microwave
conductivity. We find that such a decrease accounts for the observed increase;
a study of these different phenomena over a wide range of high Tc materials
would confirm this correlation.
