1. M. Tinkham,Phys. Rev. 129, 2413 (1963).
2. G. J. Dolan and J. Silcox, inLow Temperature Physics LT-13 (Proc. 13th Int. Conf. Low Temp. Phys., Boulder, 1972) (Plenum, New York, 1972).
3. G. J. Dolan and J. Silcox,Phys. Rev. Letters 30, 603 (1973).
4. R. J. Watts-Tobin and G. M. Waterworth, to be published.
5. R. J. Watts-Tobin, private communication.