 Solution

Condition on the molecule being empty. Its instantaneous incoming-charge jump rate is $f_1$, so the unconditional stationary number of arrivals per unit time isThis stationary jump traffic at a single contact is a unidirectional jump count, not a signed net current. Stationarity also gives $R_{\mathrm{out}}=f_4{p}_A^{\mathrm{st}}=R_{\mathrm{in}}$. Multiplying their difference by the charge gives zero net stationary electrical current through the molecule's single contact, even though individual arrival and departure events continue.