I have noticed that I can't squat ATG until or unless I hold onto something in front of me.I tend to fall backwards as soon as I tend to go parallel to floor or more.I do increase the depth if I hold a dumbbell I'm front or widen my stance.Is it a mobility issue and I think it's hampering my leg growth.
Probably a lack of ankle mobility. You can use weightlifting shoes to compensate it, but that won't fix the problem, just allow you to squat in a safe position with your current mobility
You should foam roll and stretch your calves. You can do deep squats for 2-3 minutes at a time focusing on your ankles (and add a plate or something under the front part of your feet to increase the stretch if you want to), and/or do that: http://myfiveminuteyoga.com/wp-content/uploads/2012/10/feet-to-bricks.jpg
Also make sure your ankles are warmed up before squatting, foam rolling, or stretching
I’m working on building up my squatting ability right now — among other things — using body-weight exercises. BJ Gaddour has an excellent book that is super cheap via Kindle. It is called “Your Body is Your Dumbbell.” That’s what I’m using. For squats, he has you start out by squatting with your back against a wall. He teaches correct positioning, such as ensuring your feet are making three-point connection with the ground: heel, big toe, little toe. He also teaches how to make his exercises progressively more difficult. Really can’t recommend his approach enough.