Dedicated wiring to luminaire - what's the minimum gauge
You're logicking out that since the lamp can't draw more than 100W, a smaller wire size should suffice.  That is covered under the tap rules.     

Metal conduit is required because the lamp could suffer a fault condition where it pulls too much current for its tap wiring but not enough to trip the breaker.  For instance if you had a 14AWG tap off a 125A circuit, and then had a 75A fault.  

But this is all pointless if you can't find 16/18/20 AWG THHN which is actually cheaper than 14 AWG, and you probably can't.   

Not least, because NEC does not allow wiring smaller than 14 AWG on any branch circuit.  Some cities have local ordinances increasing it to 12 AWG. 

Within equipment is a different story, e.g. the interconnections within a chandelier.