oye look.........first of all angular momentum is conserved abt the pt of contact..........ill tell u how..................

all forces comming into picture.....the infinite normal force.......the infinite frictional force........are acting at the pt of contact.....so these forces will not produce any torque abt the pt of contact as the moment arm(or watevr u call it) length =0...............hence the net external torque abt that pt =0.............nudge me if u have further doubts.........