1. G. Ahlers, in The Physics of Liquid and Solid Helium, Part 1, K. H. Bennemann and J. B. Ketterson, eds. (Wiley, New York, 1976), Chapter 2, p. 85.
2. R. B. Griffiths and J. C. Wheeler, Phys. Rev. A2, 1047 (1970).
3. E. K. Riedel, Phys. Rev. Lett. 28, 675 (1972).
4. C. A. Gearhart, Jr. and W. Zimmermann, Jr., to be published;
5. C. A. Gearhart, Jr., Thesis, University of Minnesota (1977), available from University Microfilms International, Ann Arbor, Michigan.