As Eric said, no limits as far as how low... but take a look at this picture from the official field from Kickoff in New Hampshire.
That is at the end of the Lane Divider wall and we see a separate piece of carpet on top of the metal plates that are stabilizing the Lane wall itself.
It is about 1/4" to 1/2" above the regular field carpet so keep that in mind especially if you plan on racing around & especially hugging the inside turns or inside wall as it may be.
Here's another view of that from higher and off to the left.
