calculate the interconnect driver output impedence

I want to design a transmission line on chip,
and i am now trying to calculate the driver output impedance, to match it with the transmission impedance.
i used two method:
the first one: used ohm's law and Vrms/Irms, where got on a huge value approx. 350M ohm.
the 2nd: set the vdd and vin to 0, and applied a current source 1A between the output and gnd, the output was a voltage waveform take the rms (=-average), it was 11.14, which equal to impedance.

which way is correct, because there is a great difference. i am using cadence.
thank in advance