The importance of friction in the description of low-temperature   dephasing
  We discuss the importance of the real part of the Feynman-Vernon influence action for the analysis of dephasing and decay near the ground state of a system which is coupled to a bath. Using exactly solvable linear quantum dissipative systems, it is shown how the effects of the real and the imaginary part (describing friction and fluctuations, respectively) may cancel beyond lowest-order perturbation theory. The resulting picture is extended to a qualitative discussion of nonlinear systems and dephasing of degenerate fermions. We explain why dephasing rates will, in general, come out finite at zero temperature if they are deduced from the imaginary part of the action alone, a procedure which is reliable only for highly excited states. 
Florian Marquardt