1. Cavendish Laboratory, Cambridge University, Madingley Road, Cambridge CB3 0HE, U.K., Department of Physics and Astronomy, University of Edinburgh, James Clerk Maxwell Building, King's Buildings, Mayfield Road, Edinburgh EH9 3JZ, U.K., and Max-Planck-Institut für Polymerforschung, Postfach 3148, D-55021 Mainz, Germany