  We consider a wave propagating through a thin disordered slab inside a wire
or waveguide of finite width. In the dense weak scattering limit, the
statistics for the complex reflection and transmission coefficients (the
coherent field) is found to depend dramatically on the contribution of
evanescent modes or closed channels, leading to an effective refractive index
whose real part is quite sensitive to the closed channels inclusion. In
contrast, evanescent modes play no role in the statistical average of
transmittances and reflectances. The theoretical predictions, based on the
perturbative Born series expansion, are in excellent agreement with numerical
simulations in disordered wires.
