1. L.E.E.E., E.A.387, Universite´ de Paris X Nanterre, 1, Chemin Desvallie`res, 92410 Ville d’Avray, France Tel.: +33-1-47-09-70-13; Fax: +33-1-47-09-16-45
2. L.P.M.M., U.M.R. C.N.R.S. n°7554, I.S.G.M.P., Universite´ de Metz, Ile du Saulcy, 57045 Metz, France