1. Department of Physics, Chemnitz University of Technology, 09107 Chemnitz, Germany, IFW Dresden, P.O. Box 270116, D-01171 Dresden, Germany, Institute for Nanotechnology, Forschungszentrum Karlsruhe, Postfach 3640, D-76021 Karlsruhe, Germany, and Department of Electrical Engineering, Princeton University, Princeton, New Jersey 08544