1. For a review see: T. Ando, A. B. Fowler, and F. Stern, Rev. Mod. Phys. 54, (1982).
2. A. B. Fowler et al., Phys. Rev. Lett. 16, 901 (1966).
3. V. M. Pudalov, S. G. Semenchinskii, and V. S. Edel’man, JETP Lett. 39, 576 (1984).
4. S. V. Kravchenko et al., Phys. Rev. B 50, 8039 (1994). S. V. Kravchenko et al., Phys. Rev. B 51, 7038 (1995).
5. S. Das Sarma and E. H. Hwang, http://xxx.lanl.gov/abs/cond-mat/9812216 http://xxx.lanl.gov/abs/cond-mat/9901117.