Handles.DrawWireArc
static function DrawWireArc(center: Vector3, normal: Vector3, from: Vector3, angle: float, radius: float): void;
static void DrawWireArc(Vector3 center, Vector3 normal, Vector3 from, float angle, float radius);
static def DrawWireArc(center as Vector3, normal as Vector3, from as Vector3, angle as float, radius as float) as void
Parameters

center The center of the circle.
normal The normal of the circle.
from The direction of the point on the circle circumference, relative to the center, where the arc begins.
angle The angle of the arc, in degrees.
radius The radius of the circle

Note: Use HandleUtility.GetHandleSize where you might want to have constant screen-sized handles.
Description

Draw a circular arc in 3D space.


Wire Arc in the Scene View.
	// Create a 180 degrees wire arc with a ScaleValueHandle attached to the disc
	// that lets you modify the "shieldArea" var in the WireArcExample.js
	
	@CustomEditor (WireArcExample)
	class DrawWireArc extends Editor {
	    function OnSceneGUI () {
	    	Handles.color = Color.red;
	        Handles.DrawWireArc(target.transform.position, 
				target.transform.up, 
				-target.transform.right, 
				180, 
				target.shieldArea);
	        
	        target.shieldArea = 
			Handles.ScaleValueHandle(target.shieldArea,
	        				target.transform.position + target.transform.forward*target.shieldArea,
	        				target.transform.rotation,
	        				1,
	        				Handles.ConeCap,
	        				1);
	    }
	}
And the script attached to this Handle:
	// WireArcExample.js
	
	var shieldArea : float = 5;