1. L. Esaki and R. Tsu, Internal Report RC 2418, IBM Research, March 26 (1969).
2. R. Dingle, H. L. Stormer, A. C. Gossard, and W. Wiegmann, Appl. Phys. Lett. 37 805 (1978).
3. F. Stern, Phys. Rev. B 5, 4891 (1972).
4. T. Ando, A. B. Fowler, and F. Stern, Rev. Mod. Phys. 54 437 (1982).
5. A. B. Fowler, F. F. Fang, W. E. Howard, and P.J. Stiles, Phys. Rev. Lett. 16, 901 (1966).