  Using a full dispersion description of phonons, the thermal conductivities of
bulk Si and Bi2Te3 are evaluated using a Landauer approach and related to the
conventional approach based on the Boltzmann transport equation. A procedure to
extract a well-defined average phonon mean-free-path from the measured thermal
conductivity and given phonon-dispersion is presented. The extracted
mean-free-path has strong physical significance and differs greatly from simple
estimates. The use of simplified dispersion models for phonons is discussed,
and it is shown that two different Debye temperatures must be used to treat the
specific heat and thermal conductivity (analogous to the two different
effective masses needed to describe the electron density and conductivity). A
simple technique to extract these two Debye temperatures is presented and the
limitations of the method are discussed.
