Right now I am running this on a layout that has one sub-cell:

load folded_cascode_p_in.mag
box 0 0 0 0
flatten folded_cascode_p_in_extracted
load folded_cascode_p_in_extracted
box 0 0 0 0 
port ibias index 1
port in_m index 2 
port in_p index 3
port out index 4
port vdd index 5
port vss index 6

extract all
ext2sim labels on
ext2sim
extresist tolerance 10
extresist
ext2spice lvs
ext2spice cthresh 0.01
ext2spice extresist on
ext2spice