Arn, 3 bytes

phi 

Just a builtin, too short to be compressed. Returns 1.61803398874989484820458683436563811.

A more interesting one (returns 1.61803398874989484821):

6 bytes

l[├Qn0 

Try it!

Explanation

Unpacked: :-1+:/5

:- Halve 1 Literal one + Plus :/ Square root of 5 Literal five