1. L. V. Butov, A. Zrenner, G. Abstreiter, G. Bohm, and G. Weimann, Phys. Rev. Lett. 73, 304 (1994).
2. U. Sivan, P. M. Solomon, and H. Strikman, Phys. Rev. Lett. 68, 1196 (1992).
3. M. Bayer, V. B. Timofeev, F. Faller, T. Gutbrod, and A. Forchel, Phys. Rev. B 54, 8799 (1996).
4. Yu. E. Lozovik and V. I. Yudson, JETP Lett. 22, 26 (1975); Zh. Éksp. Teor. Fiz. 71, 738 (1976) [Sov. Phys. JETP 44, 389 (1976)]; Solid State Commun. 18, 628 (1976); Solid State Commun. 21, 211 (1977); Yu. E. Lozovik, Proceedings of the First All-Union Conference on Dielectric Electronics [in Russian], Tashkent, 1973.
5. A. V. Klyuchnik and Yu. E. Lozovik, Zh. Éksp. Teor. Fiz., 76, 670 (1979) [Sov. Phys. JETP 49, 335 (1979)]; J. Low Temp. Phys. 38, 761 (1980); J. Phys. C 11, L483, (1978); I. O. Kulik and S. I. Shevchenko, Solid State Commun. 21, 409 (1977); Yu. E. Lozovik and V. I. Yudson, Solid State Commun. 22, 117 (1977); Yu. E. Lozovik and A. V. Poushnov, Phys. Lett. A 228, 399 (1997).