  We show that the nonperturbative quantum transport equations, the
`Kadanoff-Baym equations', can be understood as the ensemble average over
stochastic equations of Langevin type. For this we couple a free scalar boson
quantum field to an environmental heat bath with some given temperature T. The
inherent presence of noise and dissipation related by the
fluctuation-dissipation-theorem guarantees that the modes or particles become
thermally populated on average in the long-time limit. This interpretation
leads to a more intuitive physical picture of the process of thermalization and
of the interpretation of the Kadanoff-Baym equations.
