1. G. V. Chester,Lectures in Theoretical Physics, Vol. XI-B, K. T. Mahanthappa and W. E. Britten, eds. (Gordon and Breach, New York, 1969), p. 253; see alsoPhys. Rev. A2, 256 (1970).
2. A. F. Andreev and I. M. Lifshitz,Zh. Eksperim. i Teor. Fiz. 56, 2057 (1969) [English transl.,Soviet Phys.?JETP 29, 1107 (1969)].
3. A. J. Leggett,Phys. Rev. Letters 25, 1543 (1970).
4. R. A. Guyer,Phys. Rev. Letters 26, 174 (1971).
5. W. Mullin,Phys. Rev. Letters 26, 611 (1971);Phys. Rev. A3, 1247 (1971).