1. See for instance: Vaks, V.G., Larkin, A.I., Pikin, S.A.: Zh. Eksp. Teor. Fiz.53, 1089 (1967) [Sov. Phys.-JETP26, 647 (1968)]
2. Brezin, E., Wallace, D.J., Wilson, K.G.: Phys. Rev. B7, 232 (1973)
3. Wallace, D.J., Zia, R.K.: Phys. Rev. B12, 5340 (1975)
4. Nelson, D.R.: Phys. Rev. B13, 2222 (1976)
5. Schäfer, L., Horner, H.: Z. Phys. B-Condensed Matter29, 251 (1978)