  Through an exact analysis using quantum Langevin dynamics, we demonstrate the
crossover from ballistic to diffusive thermal transport in a harmonic chain
with each site connected to Ohmic heat reservoirs. The temperatures of the two
heat baths at the boundaries are specified from before whereas the temperatures
of the interior heat reservoirs are determined self-consistently by demanding
that in the steady state, on average, there is no heat current between any such
(self-consistent) reservoir and the harmonic chain. Essence of our study is
that the effective mean free path separating the ballistic regime of transport
from the diffusive one emerges naturally.
