I haven't used the L293s (though I have some here to test at some point), but I'd be very cautious about killing them (not to mention the motor). How are you driving it? Is there current limiting in place? I hope so because 12v through 7ohm = 1.7A, so you either need some big (power wasting) resistor in place or some fancier current limiting. Hopefully someone who knows more can chime in.