  We investigate the role of the quartic anharmonicity in lattice dynamics and
thermal transport of type-I clathrate Ba$_{8}$Ga$_{16}$Ge$_{30}$ based on
\textit{ab initio} self-consistent phonon calculations. We show that the strong
quartic anharmonicity of rattling guest atoms causes the hardening of
vibrational frequencies of low-lying optical modes and thereby affects
calculated lattice thermal conductivities $\kappa_{L}$ significantly, resulting
in an improved agreement with experimental results including the deviation from
$\kappa_{L}\propto T^{-1}$ at high temperature. Moreover, our static
simulations with various different cell volumes show a transition from
crystal-like to \textit{glasslike} $\kappa_{L}$ around 20 K. Our analyses
suggest that the resonance dip of $\kappa_{L}$ observed in clathrates with
large guest-free-spaces is attributed mainly to the strong Umklapp scattering
of acoustic modes along with the presence of higher-frequency dispersive
optical modes.
