Although his swing looks a little loopy Jim Furyk is a great example of a one plane swing, he takes the club outside coming back but his delivery position is exactly on one plane.
Getting full mobility in your wrist extensor is important because it allows your wrists to move properly in the backswing so you do not have an open face at the top of your backswing
Many slicers hit the ball to the right simply because they are lined up to the right, when they go to hit the ball their mind knows the target is to the left and then subconsciously changes the swing to outside in so that the club travels towards the target causing a slice