2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/27492These are the lecture notes to be published in the Proceedings of XXXI Rencontres de Moriond conference ``Correlated Fermions and Transport in Mesoscopic Systems'', (20-27 January 1996, Les Arcs, France). The notes are based on two recent papers [D L. Maslov and M. Stone, Phys. Rev. B 52, R5539 (1995), and D. L. Maslov, ibid., R14368 (1995)].10 pages, 2 figures are encluded as encapsulated postscript files. File epsfig.sty is included in the packageCondensed Mattertext