  We consider quantum interferences of classically allowed or forbidden
electronic trajectories in disordered dielectrics. Without assuming a directed
path approximation, we represent a strongly disordered elastic scatterer by its
transmission matrix ${\bf t}$. We recall how the eigenvalue distribution of
${\bf t.t}^{\dagger}$ can be obtained from a certain ansatz leading to a
Coulomb gas analogy at a temperature $\beta^{-1}$ which depends on the system
symmetries. We recall the consequences of this random matrix theory for
quasi--$1d$ insulators and we extend our study to microscopic three dimensional
models in the presence of transverse localization. For cubes of size $L$, we
find two regimes for the spectra of ${\bf t.t}^{\dagger}$ as a function of the
localization length $\xi$. For $L / \xi \approx 1 - 5$, the eigenvalue spacing
distribution remains close to the Wigner surmise (eigenvalue repulsion). The
usual orthogonal--unitary cross--over is observed for {\it large} magnetic
field change $\Delta B \approx \Phi_0 /\xi^2$ where $\Phi_0$ denotes the flux
quantum. This field reduces the conductance fluctuations and the average
log--conductance (increase of $\xi$) and induces on a given sample large
magneto--conductance fluctuations of typical magnitude similar to the sample to
sample fluctuations (ergodic behaviour). When $\xi$ is of the order of the
