Low kicks to the thigh can be effective for slowing an opponent's footwork and can cause painful, and occasionally disabling, muscle spasms. However, it is generally more appropriate for combat sports than it is for life-and-death self defense situations.
For self-defense it is best to disable an opponent as quickly and decisively as possible. The knee is a much more vulnerable part of the leg, and if self-defense is your aim, I recommend the use of oblique kicks to the knee (particularly the inside of the opposite knee) in lieu of something less effective like a low kick targeting the thigh.
Opening with an oblique kick to the inside of the opponent's knee when combined with a simultaneous volley of fist and elbow strikes, can often end a fight straight away. Few people are prepared to defend their head and knee(s) at the same instance. This is even true of many martial arts practitioners.
It should take fewer strikes, with less energy, to disable an opponents knee than it would their thigh. However, kicking the knee in such a way can cause permanent ligament and soft-tissue damage. It is important to consider the stakes of the situation before you attempt to destroy someone's knee joint, but if forced into a situation where one must protect life-and-limb it is best to end the conflict as quickly as possible.