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An approach for studying the performance of phase-coherent devices under high-frequency conditions is presented. Quantum predictions on cutoff frequencies are obtained by directly solving the time-dependent Schrodinger equation under oscillating potential profiles at frequencies comparable with the inverse of the electron transit time. As an example, the small-signal admittance parameters for a simple double-gate Si transistor are computed, showing that its intrinsic amplifying properties are degraded at terahertz frequencies. Classical results, obtained by solving the classical Boltzmann equation through the standard Monte Carlo technique, are comparable to quantum predictions. The approach opens a new path for the understanding of the electron phenomenology in phase-coherent devices under ac conditions.