The possible extraction of inelastic mean free path (IMFP or lambda(i)) 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, lambda(i), as previously thought, but a length LAMBDA = [lambda(i)-1 + lambda(tr)-1)]-1/2, where lambda(tr), the transport mean free path, is the length associated with large-angle deflection. It is significant that this length LAMBDA is also the length which characterizes the overlayer experiment for an ultrathin overlayer (thickness much-less-than IMFP) and at an exit angle of approximately 45-degrees, and is also the length obtained using two-stream methods [V. M. Dwyer, Surf. Sci. 291, 261 (1993)].