1. M. Papoular, inProc. Low Temp. Phys. 14, ed: by M. Krusius and M. Vuorio (North-Holland, 1975), Vol. 1, p. 333.
2. G. Goellner, R. Behringer, and H. Meyer,J. Low Temp. Phys. 13, 113 (1973); P. Leiderer, D. R. Watts, and W. W. Webb,Phys. Rev. Lett. 33, 483 (1974).
3. G. Ahlers,Phys. Rev. Lett. 24, 1333 (1970).
4. G. Ahlers and D. S. Greywall, inLow Temperature Physics?LT13, ed. by K. D. Timmerhaus, W. J. O'Sullivan, and E. F. Hammel (Plenum, New York, 1974), Vol. 1, p. 586.
5. K. Kawasaki and J. D. Gunton,Phys. Rev. Lett. 29, 1661 (1972).