Sum Rule Anomaly from Suppression of Inelastic Scattering in the   Superconducting State
  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. 
Frank Marsiglio