1. Digital Instruments/Veeco Metrology Group, 112 Robin Hill Rd., Santa Barbara, California 93110
2. Department of Engineering Materials, University of Sheffield S1 3JD, U.K.
3. Department of Polymer Science, University of Akron, Akron, Ohio 44325
4. Laboratoire de Physique des Polymères, CP 223, Université Libre de Bruxelles, Boulevard du Triomphe, B-1050 Brussels, Belgium