1. D. Ugarte, Laboratório Nacional de Luz Síncrotron (CNPq/MCT), Caixa Postal 6192, 13083-970 Campinas SP, Brazil.
2. A. Châtelain, Institut de Physique Expérimentale, Département Physique, Ecole Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland.
3. W. A. de Heer, School of Physics, Georgia Institute of Technology, Atlanta, GA 30332, USA.