1. R. N. Gurzhi, Soviet Phys. 18, 515. Translation from Russian.
2. R. N. Gurzhi, Soviet Phys. - JETP 19, 490. Translation from Russian.
3. M. Roesler, Phys. Status Solidi 7, K75. In German.
4. S. H. Charap, Phys. Rev. Letters 13, 237.
5. D. C. McCollum, J. Callaway and R. L. Wild, Phys. Rev. 136, A426.