  We consider a two-level system coupled to a mesoscopic two-terminal conductor
that acts as measuring device. As a convenient description of the conductor we
introduce its scattering matrix. We show how its elements can be used to
calculate the relaxation and decoherence rates of the two-level system. Special
emphasis is laid on the charge screening in the conductor that becomes
important in the many-channel limit. Finally we give some examples that
illustrate charge screening in different limits.
