We study the signatures of a classical mixed phase space for open quantum
systems. We find the scaling of the break time up to which quantum mechanics
mimics the classical staying probability and derive the distribution of
resonance widths. Based on these results we explain why for mixed systems two
types of conductance fluctuat ions were found: quantum mechanics divides the
hierarchically structured chaotic component of phase space into two parts - one
yields fractal conductance fluctuations while the other causes isolated
resonances. In general, both types appear together, but on different energy
scales.Comment: restructured and new figure