The frequency dependence of the current-current fluctuations is expressed in terms of energy-dependent amplitudes of the scattering matrix of the conductor. The frequency-dependent noise spectrum at equilibrium depends on the reflection amplitudes of the conductor. The theory is illustrated by considering a number of differing conductors with long-lived states. Special attention is paid to the case of a double-barrier structure in the presence of a current. © 1992 The American Physical Society.