2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/27341A non-perturbative random-matrix theory is applied to the transmission of a monochromatic scalar wave through a disordered waveguide. The probability distributions of the transmittances T_{mn} and T_n=\sum_m T_{mn} of an incident mode n are calculated in the thick-waveguide limit, for broken time-reversal symmetry. A crossover occurs from Rayleigh or Gaussian statistics in the diffusive regime to lognormal statistics in the localized regime. A qualitatively different crossover occurs if the disordered region is replaced by a chaotic cavity. ***Submitted to Physical Review E.***7 pages, REVTeX-3.0, 5 postscript figures appended as self-extracting archive. A complete postscript file with figures and text (4 pages) is available from http://rulgm4.LeidenUniv.nl/preprints.htmlCondensed Mattertext