POSSIBLE COHERENCE PEAK IN THE MICROWAVE CONDUCTIVITY OF YBA2CU3O7

We have measured the real and imaginary parts of the microwave conduct ivity at 5GHz of oriented powders of YBa2Cu3O7-delta. The real part si gma1 has a large, sharp peak just below T(c). Rather than treating the whole of this peak as due to coherence effects, we have used effectiv e medium theory to account for sample inhomogeneity and fluctuations. This analysis suggests that there may be a very small coherence peak o f the BCS type just below T(c), but that it is very strongly suppresse d.