Try the ZX part in the Analog Miscellaneous, ANL_MISC, library - you may have to get a later version to get this model.
Pins 1&2 are a control voltage, pin 3 is the reference, resistance in this case, to 0, pins 4&5 are a floating resistance (in this case). Generate you time varying function as a voltage and apply this as the control, the output resistance (itc) will be Rref * Vin, ensure that your control voltage remains above 0, otherwise circuit behaviour might be a little strange as things try to model negative resistance.
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