My question is how the maximum quantum contact conductance 2*q^2/h comes from. The maximum contact conductance can only be obtained when there is no scattering in the contacts between nano-channel and two big pads. (To my understanding, resistance comes from scattering, but if there is no scattering, how come we got the conductance with only 2*q^2/h, but not infinity.) How my question comes from:When a nano-channel (quantum contact point, nanowire or nanotube) (only one energy level with two spins) contact with two big pads, there is a maximum conductance 2*q^2/h. This maximum conductance can be obtained only if the transport in the nano-channel is ballistic. In other words, the conductance comes from the contacts only. (It was described "The origin of the resistance: redistribution of the current among the current-carrying modes at the interfaces"). However, people said, the maximum conductance can be got only if there is no scattering at the contacts. But what my understanding is the resistances come from scattering, if there is no scattering, why the conductance is not infinity, but only 2*q^2/h. Therefore, my understanding is that there are some scatterings at the contact between big pads with the nano-channel and that limit the conductance to be 2*q^2/h (but there is no scattering in the channel due to the ballistic transport). Am I wrong? I appreciate any comment.