  We consider a mesoscopic measuring device whose conductance is sensitive to
the state of a two-level system. The detector is described with the help of its
scattering matrix. Its elements can be used to calculate the relaxation and
decoherence time of the system, and determine the characteristic time for a
reliable measurement. We derive conditions needed for an efficient ratio of
decoherence and measurement time. To illustrate the theory we discuss the
distribution function of the efficiency of an ensemble of open chaotic
cavities.
