1. J. K. Gimzewski, R. R. Schlittler, V. Langlais, IBM Research Division, Zurich Research Laboratory, Säumerstrasse 4, 8803 Rüschlikon, Switzerland. C. Joachim and H. Tang, CEMES-CNRS, 29 rue J. Marvig, Boı̂te Postale 4347, 31055 Toulouse Cedex, France. I. Johannsen, Risø National Laboratory, Condensed Matter Physics and Chemistry Department, Post Office Box 49, DK-4000 Roskilde, Denmark.