Therefore, these phonons have to possess energy of formula_116, which is a significant fraction of Debye energy that is needed to generate new phonons. The probability for this is proportional to formula_117, with formula_118. Temperature dependence of the mean free path has an exponential form formula_119. The presence of the reciprocal lattice wave vector implies a net phonon backscattering and a resistance to phonon and thermal transport resulting finite "λ"L, as it means that momentum is not conserved. Only momentum non-conserving processes can cause thermal resistance.