Note that the conductance quantum does not mean that the conductance of any system must be an integer multiple of G 0. Instead, it describes the conductance of two quantum channels (one channel for spin up and one channel for spin down) if the probability for transmitting an electron that enters the channel is unity, i.e. if transport through the channel is ballistic. If the transmission probability is less than unity, then the conductance of the channel is less than G 0. The total conductance of a system is equal to the sum of the conductances of all the parallel quantum channels that make up the system.