The cascode current mirror looks like the following.
R1 and Vdc are not part of the current mirror circuit, they are part of the test circuit. Vdc will be swept from 0 volts to 6 volts in order to find the output voltage swing capable in this circuit. This example uses a current marker and a voltage marker.
Choose Setup... under the Analysis menu and click the button. You should see the following dialog.
When Probe loads it will show plots of current through R1 and voltage on the drain of M4. The scale of the two signals is quite different and the current plot appears as though there is 0 amps through R1.
Delete the R1 current plot by clicking on the label -I(R1) next to the red square at the bottom of the Probe window. Select Add Y Axis under the Plot menu. This will add another Y Axis to the Probe window. Add a trace with the Trace Expression: -I(R1). this will add the R1 current plot to the plot window. Your plot should now look like the following.
The minimum output voltage is approximately 0.8V.
Another DC Sweep example using a Wide Swing Cascode Current Mirror