pplane is not part of MATLAB, so it must be a package from the file exchange, or some other source. You might want to contact the author with questions.
You raise a number to a fractional power like this:
Why? Exponentiation has a higher order of precedence than division. So when you wrote n^2/3, MATLAB sees n^2, and recgnizes that exponentation comes first in the list. So it squares n, and THEN divides the result by 3.
Since you want MATLAB to raise it to the (2/3) power, just use parens as I show.