measurement of output impedance of a CMOS inverter
You should really draw the circuit you're using to simulate this as you need to get the inverter biased at the right operating point by shorting the input to output via a resistor and removing any AC from the input. From the netlist it looks like you've done this correctly.

But then you seem to apply a DC signal at the input of 0 Volt: 

VSIG IN VSS 0

That's no good!

I would use this:







simulate this circuit &ndash; Schematic created using CircuitLab

Then do an AC analysis over frequency.
As Zout = Vout/Itest  and Itest = 1 (due to AC magnitude = 1 of Itest)
You can just plot the magnitude of V(out) and that will be the output impedance.