  In voltage- and temperature-biased coherent conductors quantum screening
effects occur if the conductor's transmission is energy-dependent. Here, we
show that an additional ac-driven terminal can act as a probe for a direct
readout of such effects, hitherto unexplored. We find that screening of charges
induced by the static biases impacts already their standard linear
thermoelectric response coefficients due to nonlinear effects when accounting
for the frequency of the time-dependent driving. Those effects should be
observable under realistic experimental conditions and can literally be
switched on and off with the ac-driving.
