The motion of a particle in a ring of length L is influenced by a dirty metal environment whose fluctuations are characterized by a short correlation distance L. We analyze the induced decoherence process, and compare the results with those obtained in the opposing Caldeira–Leggett limit ( L). A proper definition of the dephasing factor that does not depend on a vague semiclassical picture is employed. Some recent Monte Carlo results about the effect of finite temperatures on 'mass renormalization' in this system are illuminated.