Thevenin Equivalent Circuit of Two-Port (Transmission Matrix)
Vth/V1 = 1/A 
and
Vth/I1 = 1/C
This means that the thevenin voltage is either 1/A or 1/C depending on the
source your using, a voltage source (V1) or a current source (I1), at the input
terminals.
The network does not have any source term and that needs to be applied at the input, thus Vthev depends on the source impedance. These two case are a perfect
voltage source (V1) or a perfect current source (I1).