  The ac Harman method has been used for the direct estimation of dimensionless
thermoelectric figure of merit (zT) through ac/dc resistance measurements.
However, accurate zT estimation with a four-probe configuration is difficult
owing to the occurrence of a thermal phase-delay in the heat flow with a low
frequency current. This study reports an exact solution for zT estimation by
solving the heat conduction equation. The analysis can explain the reverse heat
flow, which is the main source of the error in the four-probe configuration,
and the experimentally obtained behavior of the frequency dependence of zT of
(Bi,Sb)$_2$Te$_3$. Approximately 20 % of the error is caused by a thermal
phase-delay, unless an appropriate current frequency and voltage-terminal
position are chosen. Thus, an accurate zT evaluation using a four-probe
configuration at any voltage terminal position is achieved. These findings can
lead to interesting thermoelectric metrology and could serve as a powerful tool
to search for promising thermoelectric materials.
