  Utilizing atomistic lattice dynamics and scattering theory, we study thermal
transport in nanodevices made of 10 nm thick silicon nanowires, from 10 to 100
nm long, sandwiched between two bulk reservoirs. We find that thermal transport
in devices differs significantly from that of suspended extended nanowires, due
to phonon scattering at the contact interfaces. We show that thermal
conductance and the phonon transport regime can be tuned from ballistic to
diffusive by varying the surface roughness of the nanowires and their length.
In devices containing short crystalline wires phonon tunneling occurs and
enhances the conductance beyond that of single contacts.
