The possible extraction of inelastic mean free path (IMFP or λi) data from elastic backscattering experiments is analyzed within the transport approximation. It is found that the characteristic length obtained by this method is not the IMFP, λi, as previously thought, but a length Λ=[λ−1i(λ−1i+λ−1tr)]−1/2, where λtr, the transport mean free path, is the length associated with large‐angle deflection. It is significant that this length Λ is also the length which characterizes the overlayer experiment for an ultrathin overlayer (thickness≪IMFP) and at an exit angle of ∼45°, and is also the length obtained using two‐stream methods [V. M. Dwyer, Surf. Sci.291, 261 (1993)].