Scattering by one-dimensional smooth potentials: between WKB and Born   approximation
  The paper discusses the applicability of WKB and Born (small perturbations) approximations in the problem of the backscattering of quantum particles and classical waves by one-dimensional smooth potentials with amplitudes small compared to the energy of the incident particle (above-barrier scattering). Both deterministic and random potentials are considered. The dependence of the reflection coefficient and localization length on the amplitude and the longitudinal scale of the scattering potential is investigated. It is shown that perturbation and WKB theories are inconsistent in the above-barrier backscattering problem. Not only the solutions but the regions of validity of both methods as well depend strongly on the details of the potential profile, and are individual for each potential. A simple criterion that allows determining the boundary between the applicability domains of WKB and Born approximations is found. 
K.Yu. Bliokh, V.D. Freilikher, and N.M. Makarov