[1] Dept. of Electronics and Elec. Eng., University of Glasgow, Glasgow G12 8LT, United Kingdom
[2] Institute of Solid-State Physics, Technical University of Berlin, Berlin D-10623, Germany
[3] A.F. Ioffe Phys.-Technical Institute, St. Petersburg 194021, Russia
[4] Department of Physics, University of Surrey, Guildford GU2 5XH, United Kingdom