(1) Eindhoven University of Technology, Department of Physics, P.O. Box 513, 5600 MB Eindhoven, The Netherlands (Fax: +31-40/2456050, E-mail: a.c.den.boer@phys.tue.nl, h.c.w.beijerinck@phys.tue.nl, k.a.h.leeuwen@phys.tue.nl), NL
Abstract:
Received: 1 April 1996/Revised version: 24 September 1996