Within the framework of the theory of random walk [1] and following a Landauer-like formalism [2], we derive an analytical expression for the global transmission coefficient which defines the fraction of the total heat that is transported by ballistic phonons from the hot to the cold reservoirs in the presence of defects, impurities, grain boundaries, interfaces…. This transmission coefficient depends only on probabilities p (forward scattering), q (backward scattering, r (absorption) and N, (number of scattering centers), regardless of phonon frequencies and wavelengths [5]. Impurities, defects, grain boundaries, interfaces… constituent sites for scattering (called scattering centers SCs). These sites are modelized as multiple screens at which 3 processes are possible :-forward scattering with probability p-backward scattering with probability q-absorption with probability r Screens divide the nanostructure into cells.