I haven't grown a nectarine in a bush form, but I've grown its cousin the peach tree from seed and it is growing in a bushy form.
This tree came up by itself, probably planted by a squirrel.
It has never been pruned, and has a naturally healthy form.
It grew over 6 feet in its first year of growing, and it has started developing lots of new young side branches.
Hopefully we'll get some tasty peaches next year, less than 3 years after the tree was planted!
It's looks a lot more healthy right now that my older peach trees that were pruned.
Here's a video I made of the tree.