1. R. Berger and H. P. Lang, IBM Research Division, Zürich Research Laboratory, CH-8803 Rüschlikon, Switzerland, and Institute of Physics, University of Basel, CH-4056 Basel, Switzerland.
2. E. Delamarche, Ch. Gerber, J. K. Gimzewski, IBM Research Division, Zürich Research Laboratory, CH-8803 Rüschlikon, Switzerland.
3. E. Meyer and H.-J. Güntherodt, Institute of Physics, University of Basel, CH-4056 Basel, Switzerland.