Transmission coefficient is an unitless quantity, being defined per a lateral unit cell, i.e., doubling the lateral unit cell would double the transmission value, just because the cross section is doubled, so more current can path through the conductor.
In QuantumATK, transmission can be defined, e.g., as total (sum) transmission or transmission per spin. The total one is just a sum of the transmission coefficients for spin up and down.
One can also compute the current from the Transmission Spectrum, see the manual at
https://docs.quantumatk.com/manual/Types/TransmissionSpectrum/TransmissionSpectrum.html, where you may also find different options on how transmission can be computed.
Note that the current is computed per lateral unit cell as well, so you have to divide it by the cross section (lateral unit cell) area to get the current density, if needed.
