  We show that the perturbative expansion of the two-level correlation
function, $R(\omega)$, in disordered conductors can be understood
semiclassically in terms of self-intersecting particle trajectories. This
requires the extension of the standard diagonal approximation to include pairs
of paths which are non-identical but have almost identical action. The number
of diagrams thus produced is much smaller than in a standard field-theoretical
approach. We show that such a simplification occurs because $R(\omega)$ has a
natural representation as the second derivative of free energy $F(\omega)$. We
calculate $R(\omega)$ to 3-loop order, and verify a one-parameter scaling
hypothesis for it in 2d. We discuss the possibility of applying our ``weak
diagonal approximation'' to generic chaotic systems.
