  The decoherence of a test particle interacting with an ideal gas is studied
by the help of the effective Lagrangian, derived in the leading order of the
perturbation expansion and in order $\ord{\partial^2_t}$. The stationary
decoherence time is found to be comparable to or longer than the diffusion
time. The decoherence time reaches its minimal value for classical, completely
decohered environment, suggesting that physical decoherence is slowed down as
compared with diffusion by the quantum coherence of the environment.
