voltage drop alternative to resistor
I presume this would be for a power supply?  If the voltage drop from 12v to 6v releases too much heat, then I would recommend using a DC to DC converter of some sort.  A linear regulator is basically just a glorified resistor, so converting 12V to 6V would make a linear converter 50% efficient.  A switching power supply can be 70% to 90+% efficient, and as a result will produce far less heat.  A buck power supply would be the way to go, and you can either build one yourself with an off-the-shelf controller chip or just buy a complete module.