All this is absolutely correct.
I just want to add a note, that as ZH already said, the keyword here is coherent transport, which is the regime where the Landauer-Büttiker formalism is valid.
"Scattering" can mean many things, and while ATK does not include any of the inelastic (or elastic, for that matter) listed above, there is still a concept of scattering since the transmission probability of each individual scattering channel may be, and usually is, less than 1.
If you have diffusive transport and remove all scattering centers, phonons, etc, your conductance will go to infinity. This is the "classical zero temperature limit" (which is wrong, as we know). In ballistic transport, conductance is a transmission process, and hence has an upper bound, set by the conductance quantum time the number of modes.
The ballistic regime is obtained when the device size is smaller than the mean free path and the phase decoherence length.
For a more technical presentation, I also recommend another book by Datta, "Electronic Transport in Mesoscopic Systems".
