We thank Dr. G. Hill for carrying out the optical lithography;
and M.J. Fice for his help in constructing the e-beam lithography machine and for the use of his Monte Carlo programs. We are very grateful to Dr. D. Andrews and Dr. G.J. Davies of British Telecom for providing the high mobility material used in this work. One of us (C.J.B.F.) acknowledges an SERC studentship;