  At high temperature the infrared modes of a weakly coupled quantum field
theory can be treated nonperturbatively in real time using the classical field
approximation. We use this to introduce a nonperturbative approach to the
calculation of finite-temperature spectral functions, employing the classical
KMS condition in real time. The method is illustrated for the one-particle
spectral function in a scalar field theory in 2+1 dimensions. The result is
compared with resummed two-loop perturbation theory and both the plasmon mass
and width are found to agree with the analytical prediction.
