  We study the distribution of resonance widths P(G) for three-dimensional (3D)
random scattering media and analyze how it changes as a function of the
randomness strength. We are able to identify in P(G) the system-inherent
fingerprints of the metallic, localized, and critical regimes. Based on the
properties of resonance widths, we also suggest a new criterion for determining
and analyzing the metal-insulator transition. Our theoretical predictions are
verified numerically for the prototypical 3D tight-binding Anderson model.
