  Type-I clathrate compounds with off-center guest ions realize the
phonon-glass electron-crystal concept by exhibiting almost identical lattice
thermal conductivities $\kappa_{\rm L}$ to those observed in network-forming
glasses. This is in contrast with type-I clathrates with on-center guest ions
showing $\kappa_{\rm L}$ of conventional crystallines. Glasslike $\kappa_{\rm
L}$ stems from the peculiar THz frequency dynamics in off-center type-I
clathrates where there exist three kinds of modes classified into extended(EX),
weakly(WL) and strongly localized(SL) modes as demonstrated by Liu $et.\,al.$,
Phys. Rev. B $\textbf{93}$, 214305(2016). Our calculated results based on the
hopping mechanism of SL modes via anharmonic interactions show fairly good
agreement with observed ${\it T}$-linear rise of $\kappa_{\rm L}$ above the
plateau. We emphasize that both the magnitude and the temperature dependence
are in accord with the experimental data of off-center type-I clathrates.
